A vehicle underbody includes a floor member forming a floor portion of a vehicle, a pair of side frames respectively extending along side edges of the floor member along a longitudinal axis of the vehicle, and a cross member extending along a transverse axis of the vehicle to connect the pair of the side frames. At a center portion along the transverse axis of the vehicle, the floor member includes a ridge-shaped tunnel portion which protrudes upwards and extends along the longitudinal axis of the vehicle. The tunnel portion extends from a front portion to the cross member.
JP H02-56787U discloses a structure in which a tunnel portion (a backbone 1a) of a floor member (a vehicle floorpan 1) and a cross member (a rear sheet cross member 5A) is connected with a reinforcing member (a reinforcing member 5a). It should be noted that the members and reference numerals in parentheses refer to those in JP H02-56787U and do not relate to those used in the embodiments of the present disclosure.